This invention is directed to microphones of the dynamic or moving-coil type and to the method of making them.
It is traditional in such microphones to employ AlNiCo magnet structure and small area or diameter diaphragms. A small area diaphragm is beneficial because it means that both the diaphragm and voice coil attached to it present low mass so that the microphone is relatively insensitive to handling or to shock which could produce spurious noise. At the same time, a small diameter voice coil is compatible with AlNiCo magnets which require a high ratio of height-to-diameter (i.e., a "cylindrical" shape) in order to avoid the serious effects of self-demagnetization. Unfortunately, AlNiCo magnet designs also have a low level of flux density in the voice coil gap so that these traditional microphones have inferior acoustic sensitivity compared with modern microphones such as the "condenser" designs. Even if one were to increase the diameter of the AlNiCo magnet to improve the acoustic sensitivity, the penalties paid in increased magnet and consequent microphone sizes, not to mention the penalties paid in increased masses due to increased sizes of both the diaphragm and the voice coil, plus the penalty paid in increased mass due to the need to stiffen or compensate for the increased diameter of the diaphragm, render that approach impractical.
Thus, it will be seen that features of both the diaphragm and of the magnetic circuitry play interrelated and mutually incompatible if not mutually exclusive parts in the efficacy of dynamic microphones insofar as improvements in their acoustic sensitivity and decrease in their handling or shock sensitivity are concerned. The prime function of a microphone diaphragm is to act as a receptor for acoustic pressure waves and to convert such waves into physical force or motion at the attached transducer, in this case the voice coil in its magnetic air gap. The diaphragm must have sufficient stiffness in the plane of its major face so that it will behave as a piston and, on the other hand, the means used to support the edge of the diaphragm in the direction normal to its major face must be compliant as possible to permit easy travel of the diaphragm in such normal direction. At the same time, the diaphragm and its edge mounting must be relatively rigid radially to prevent radial motion of the voice coil and to confine its motion in the axis of the air gap. The diaphragm and its edge mounting must also be resilient enough to return the coil axially to its mid-position at the frequency and amplitude of the acoustic waves bring treated, due regard being had for the overall mass of the whole moving system.